Lower-limb prosthesis users have reduced standing balance and balance confidence. These reductions in balance confidence have been associated with reductions in social activities and quality of life. Current ankle- foot prostheses have been designed with a focus primarily on walking function, although recent studies suggest that two distinct functional modes are needed in the prosthesis to effectively mimic walking and standing tasks. Future prostheses would more closely mimic biologic function by providing a curved effective rocker shape for walking and a flat effective rocker shape for standing tasks. A prototype has been developed by the applicant that is capable of providing these two distinct modes, namely the bimodal ankle-foot system. The primary objective of this project is to determine if the standing mode, which provides a flat effective rocker shape, can improve the standing balance of veterans with lower-limb amputations compared with the curved (walking) mode. The project will involve 24 veterans with unilateral transtibial amputations. After subjects provide informed consent to participate in the project, subject-specific data will be collected and testing will occur to document light touch and vibratory sensation on the intact and residual limbs. Subjects will train to perform the balance tests of the project using their current prosthesis. After training, the subjects will walk througha hallway with their current prosthesis and their self-selected normal speed will be measured. The subjects will then perform an L-Test of functional mobility with their current prosthesis. After balance training and mobility testing with the current prosthesis is complete, the subject's residual limb socket will be removed from their current prosthesis and will be connected to the bimodal ankle-foot system. Subjects will be given time to accommodate to the bimodal ankle-foot system (set into walking mode). The bimodal ankle-foot system will be switched into standing mode and subjects will explore their functional base of support by leaning in various directions. The level of accommodation to the bimodal ankle-foot system will mimic that seen in an initial fitting at a VA prosthetics facility. Next, the subjects will perform the following balace tests with the ankle in walking and standing modes (in random order): (1) Static postural stability - Quiet standing with eyes open and eyes closed, (2) Dynamic postural stability - Limits of stability involving leaning over the forefeet and heels, and (3) Functional postural stability - Modified motor control test involving translational perturbations of the standing surface. We hypothesize that subjects' static, dynamic, and functional postural stability will be improved when using the flat effective rocker shape compared with the curved shape. Balance testing will be performed using a NeuroCom Smart Equitest Clinical Research System at the Minneapolis VA Health Care System. Lastly, walking mobility with the bimodal ankle-foot system in curved (walking) mode will be assessed, including collection of walking speed and performance of the L-Test of functional mobility, for comparison with the current prosthesis. The results of this stud will inform the further development of the bimodal ankle-foot system. It will also provide insight into this design approach for future ankle-foot prostheses and orthoses. For example, if the hypotheses of this study are supported, future powered ankle-foot prostheses and orthoses could implement similar strategies in their designs to improve standing balance of their users.